The rotational total skin electron irradiation technique at the McGill University Health Centre has undergone several developments over the past few years. Replacement of the formerly used linear accelerator has prompted many modifications to the previously reported technique. With the current technique, the patient is treated while standing on a rotating platform, with a single large field at a source to surface distance of 378 cm. The electron field is produced by a Varian 21EX linear accelerator using the commercially developed 6 MeV high dose rate total skin electron mode, along with a custom-built flattening filter. Ionization chambers, radiochromic film, and MOSFET detectors have all been used to confirm the dosimetric properties of this technique. Measurements investigating the stationary beam properties, the effects of full rotation, and the dose distributions to a humanoid phantom are reported. In addition, comparisons with commonly-used stationary beam treatment alternatives are presented.

The thesis work contains two related projects. The first project examines the characteristics of the Farmer-type ionization chamber (Farmer-type IC 2571 A) used in our experiments. An investigation was carried out to determine if the simplified "two-voltage technique" used in clinics is sufficiently accurate for saturation current estimation. Three models were used to fit the measured currents and to estimate the saturation current. The estimation was carried out by measuring the currents set-up in the Farmer-type IC while varying the applied voltages, for both polarities, for fixed and variable dose rates. The second project investigated peripheral dose in 6 MV beam using the Farmer-type IC. The signal produced in the ionization chamber when measuring the peripheral dose is 2 to 3 orders of magnitude lower than the signal produced with the chamber in the primary beam, thus, the leakage and saturation characteristics of the ionization chamber had to be investigated to ensure that they do not have adverse effects on the chamber reading. Measurements were acquired in open field (10x10 cm2) and in a dynamic MLC field (10x1 cm2), with the IC at the machine's isocenter. / Measurements allowed us to determine peripheral dose due to scatter and leakage radiation. Measurements were repeated for a blocked beam, which allow us to determine only the leakage radiation component of the peripheral dose. The two components of the peripheral dose depend strongly on the linac head configuration and shielding. Leakage radiation per MU for a dMLC field is similar or higher than the leakage radiation for a static field, which implies an increase in peripheral dose for IMRT-type treatments. Knowledge of peripheral dose can be useful in estimating dose and risk to sensitive structures outside of the primary treatment field.

Electron arc therapy is a special radiotherapeutic technique using a rotational electron beam in the treatment of large superficial tumours following curved surfaces. In those cases, arc therapy offers the best way to optimize dose uniformity while sparing healthy tissues and critical organs. The use of this technique overcomes under or over dosage problems caused by field junctions. However, electron arc therapy presents important challenges in terms of dosimetry and treatment planning. / Clinical implementation of electron arc therapy requires the study of many parameters of influence such as the radius of curvature of the treated surface, the width of the treatment field, the total angle of irradiation and the beam energy. Monitor unit calculation to deliver prescribed dose is a very critical topic and, in general, requires acquisition of a large amount of measured dosimetric data. / This project concerns the clinical implementation of electron arc therapy using an Elekta SL-25 linear accelerator in the radiation oncology department of the Maisonneuve-Rosemont Hospital (Montreal, Canada). Firstly, the objective of the study is to observe the influence of the radius of curvature, the total arc angle and the field width on the following dosimetric parameters: depth of maximum dose, isodose distributions and electron arc beam output at the depth of maximum dose. Secondly, for our particular setup, the goal is to develop a simple monitor unit calculation method, based on an analytical model fitted through measured dosimetric data covering a large range of possible clinical situations. / In order to achieve these goals, electron arc irradiations were performed on cylindrical acrylic phantoms of different radii, successively varying the total arc angle and the field width at isocentre. Results obtained with thermoluminescent dosimeters show a minor impact of the radius of curvature variation on the percent depth dose curves. However, they show a significant impact on the beam output. It was also observed that the total arc angle influences the dose at the depth of maximum dose only up to a limit angle value, different for each radius of curvature. Finally, the field width at isocentre has an impact on the beam output as well as on the bremsstrahlung contribution at the isocentre. / Concerning the monitor unit calculation, a seven parameter analytical model fitted through measured data was obtained using Origin 7 software. A relationship giving the beam output as a function of the radius of curvature and the total arc angle was found. The field width was not included in the model, but will be part of further investigation before clinical implementation. As future work, dosimetric measurements with other energies should be carried on, mainly to be able to cover a wider range of clinical cases.

Measurements of absorbed dose to water in megavoltage electron beams are carried out with cylindrical or plane-parallel ionization chambers. The perturbation factor associated with plane-parallel ionization chambers that have sufficiently large guard rings is assumed to be unity. The NACP-02 chamber is one such chamber that we have investigated in order to determine if this assumption is valid. By performing Monte Carlo simulations with the EGSnrc code, water-to-air Spencer-Attix stopping powers and perturbation factors were calculated in water with a detailed model of the NACP chamber that was validated through results obtained from measurements and simulations. These were determined for electron beam energies between 4 MeV and 19 MeV for a clinical and PSDL linear accelerator. The overall perturbation factor was found to decrease with energy at dref from 1.015 at 4 MeV to 1.004 at 19 MeV and increase with depth for a fixed beam quality.

A three dimensional, automatic, anatomy-based system for portal verification has been developed based on an FFT implementation of Pearson's correlation coefficient (PCC). The PCC requires no anatomy or point-pair identification, is robust when encountering changes in scaling and shifts in image amplitudes and requires no priori knowledge of the anatomy, which makes it an ideal candidate for portal-to-DRR image registration. Features for matching are selected from orthogonal portal images and compared to the corresponding megavoltage DRR. The position of the highest correlation value is then converted into beam-to-patient geometry and compared with the actual patient setup. By continuously generating DRRs, the system is capable of verifying translation errors, in-plane rotation and out-of-plane rotation errors. The mean accuracy of translation and rotation registrations tests were 0.58 mm and 0.79° respectively for DRR-to-DRR matching, and 1.22 mm and 1.31° respectively for portal-to-DRR matching.

The MicroPET R-4 scanner was designed for imaging small rodents such as mice and rats. In many cases its spatial resolution is not good enough to perform the required task. We have implemented an eccentric motion (commonly referred to as wobbling) which is applied to the bed during scanning. / The program which histograms the list-mode data was rewritten to increase the spatial sampling by incorporating the wobble position in the sinograms. The corrections for the dwell time, apparent crystal location, and crystal-pair efficiency are applied within the program. A series of scans were performed to decide the optimum wobble radius; it was found to be 1.50 mm. Another series of scans was performed during which a Na-22 source was moved 0.25 mm between scans with and without the optimal wobble motion. The peak-to-valley ratio between two Na-22 point sources 4.0 mm apart, improved from 1.75 in the conventional mode to 2.26 during wobbled scans applying a ramp filter. / The bed wobbling mechanism can he added to the microPET R4 or P4 scanners without any major changes to make wobble motion and without compromising any imaging modes. Implementing the wobble mechanism may present a cost-effective upgrade over a trade in or purchase of the higher performance scanner.

A clinically-useful treatment planning system for external photon beam radiotherapy must yield fast and accurate calculations of the dose distribution in the patient. The interface of the system should be "user friendly" and designed to minimize user work and errors. Visualization of the patient volume and dose calculation results should provide necessary formation without being confusing. A software module meeting these criteria has been implemented within the McGill Treatment Planning System (MPS). The MPS program is written in C code and compiled with CodeWarrior C/C++$ rm sp{TM}$ from MetroWerks Inc. The MPS program runs on the Apple Macintosh platform (either 68K or PowerPC series). This new software module permits dose calculation (modified Milan-Bentley method) and viewing of the patient volume in three dimensions. Patient anatomical data is acquired from computed tomography (CT) or magnetic resonance (MRI) images. The accuracy of the dose calculation has been verified through comparison of the calculated results with water tank and film measurements.

Space constraints often present the greatest challenge in determining the lay-out of specific radiotherapy installations. To fulfill these constraints high density concrete is often employed in construction. We have undertaken a study to establish the transmission of primary, leakage and scatter radiation through a particular type of high density concrete. In the course of this study various characteristics of leakage and scatter radiation from a 10 MV linac have also been investigated. The results of these studies are reported in this thesis. / To establish the correct chamber configuration required for the characterization of the transmission factors for primary, leakage and scatter radiation and to determine the scatter-to-incidence dose ratio a, we have measured the build-up curves and verified the validity of the inverse square law for these different radiations. The build-up data for the radiation scattered from a humanoid phantom at different angles from the primary axis have shown that even at low scatter angles little to no build-up is required for scatter radiation measurements.

In this thesis, the use of inhomogeneity corrections in intensity modulated radiotherapy (IMRT) inverse treatment planning is investigated. Firstly, the dosimetric consequences of CT beam hardening artifacts present on images used for treatment planning are estimated and found to be of little clinical significance (<1% dose difference). Secondly, experiments to evaluate the PEREGRINE Monte Carlo system (Nomos, Cranberry, PA) are undertaken for a 6 MV photon beam. The use of inhomogeneity corrections in clinical treatment planning is assessed for five clinical head and neck cancer cases. The cases are planned with the CORVUS optimization module and the dose distributions are then calculated with CORVUS and PEREGRINE in order to compare the two calculation techniques with emphasis on how each method handles tissue inhomogeneities. The plans are assessed in terms of dose, dose-volume distributions and the biological indices of TCP and NTCP. On average, PEREGRINE calculates a 1% lower mean dose to the GTV and a 2% lower mean dose to the CTV compared to the CORVUS calculations with EPL inhomogeneity corrections. In the last part of this work, quality assurance (QA) measurements are performed for a clinical case to investigate how the CORVUS and PEREGRINE calculations agree with the dose measurements on a QA phantom.

Dynamically delivered intensity modulated beams (IMBs) pose unique verification problems that may be addressed with the use of integrating continuous 3D dosimeters such as gel based Fricke dosimeters. Accurate knowledge of the ability of these dosimeters to measure adequately and precisely the delivered dose is a prerequisite for their clinical use. The magnetic properties of the ferrous and ferric ions present in the gel based Fricke dosimeter after its irradiation are the basis for the use of magnetic resonance imaging (MRI) in the measurement of dose. This thesis presents the investigation of a 3D gel based Fricke dosimetry system (Fricke-gel). A software system is developed and spin-lattice relaxation rate (R1) images are computed from MR images of irradiated Fricke-gel phantoms in order to quantify the dosimetric uncertainties resulting from the MR imaging system, from the gel itself, as well as from the external parameters. The sensitivity and the minimum detectable dose of the Fricke-gel dosimeter are determined. Validation of the dosimeter's capacity to measure dose distributions is made through measurement of percent depth dose curves (PDD's), and field profiles (open and wedged). An example of clinical utilisation of the Fricke-gel dosimeter is presented. Dose distributions are evaluated visually by 3D software tools and quantitatively analyzed by dose-volume histograms. Results show a good correlation between the Fricke-gel measured dose distributions and treatment planning software dose calculations.